1. Field of the Invention
The present invention relates to solid waste processing and, in particular, to an air classifier for light reusable materials separation from a stream of non-shredded municipal, commercial, and/or industrial solid waste.
2. Description of the Related Art
The magnitude of problems associated with the disposal of municipal, commercial, and industrial wastes is well known both from the stand point of environmental impact and costs to the community.
In order to reduce environmental impact, minimize costs and safe guard exhaustible resources, the concept of reutilizing certain components of solid waste has become more widely accepted and has been implemented in certain areas to the extent that current technology allows and subject to acceptable cost limits.
It is also known that certain components can be selected or separated from a stream of waste by means of special equipment which exploit the particular physical characteristics of the particular materials contained in the waste. One example of such special equipment is the air classifier. To date, air classification has been used to separate combustible materials from previously shredded waste.
Air classification is based on an interaction between a moving air stream and shredded solid waste which occurs within a column. The interaction process is effected by subjecting the particles to a force which is opposed to the fall of the particles under the influence of gravity. As a result, waste flow is divided into two portions. A first portion is called the "light" fraction which includes concentrated fragments of paper and cardboard as well as plastic film. The second component of the divided waste is referred to as the "heavy" fraction which mostly consists of glass, stone, and ferrous metals.
There are many air classifiers currently employed in the industry having different designs but all based on the basic principals noted above. The various air classifier utilized in the industry can be subdivided into four categories. The first is horizontal units which consist of a horizontal duct through which an air stream is directed. Shredded waste is fed to the duct and air drags the light fraction, letting the heavy fractions fall down through an outlet of the horizontal duct. Thus, such an air classifier is a pneumatic haulage system in which air speed is studied so that only the light fraction is carried by the air stream. An air classifier of this type is shown in FIG. 1.
A second type of air classifier is the vertical unit. Vertical units can have varying shapes but are substantially characterized by a vertical duct through which an air stream is directed and shredded waste is fed. Thus, the particles within the vertical duct are subjected to opposite forces due to which the heavy components of the solid waste fall towards the bottom and the light fraction follows the air stream. The vertical ducts are configured or shaped so as to increase the efficiency of the separation process. Examples of vertical ducts are shown in FIGS. 2 and 3. FIG. 2 shows a simpler version of a vertical duct whereas FIG. 3 shows a zig-zag shaped vertical duct.
The third type of air classifier is known as the inclined vibrating unit. The characteristics of inclined units are similar to the horizontal and vertical units described above. Specifically, the inclination of the unit facilitates the advance of the shredded waste through the system while the air stream entrains the light fraction. Vibrating units are similar to horizontal units except that a smaller part of air is introduced into the duct together with the waste material to be classified and most of the air is introduced below the inclined vibrating surface. An air classifier of the inclined vibratory type is shown in FIG. 4.
The fourth type of air classifier currently employed in the industry is the rotating unit. In rotating units, a primary duct in the form of a rotating cylinder receives the shredded waste and an air stream passes through the cylinder to carry away the light fraction. More particularly, the waste is fed to the cylinder from the lower part thereof by means of a belt conveyor. The air stream runs across the cylinder from its lower part to its upper part. A decantation chamber for the light fraction of the waste is disposed adjacent the upper part of the cylinder and receives the light fraction together with the air flow. The heavy fraction falls down along the cylinder slope and is discharged from the lower part of the cylinder. An example of a rotating unit is shown in FIG. 5.
Accordingly, the separation processes performed by the above-described air classifier are nothing but a rough subdivision where the light portion has a higher percentage of paper materials and the heavy portion has a higher percentage of glass, ceramics, stones, and metals. Considering that when employing such prior art classifiers, the input waste is shredded and then thoroughly mixed, such air classifiers are inappropriate for recycling i.e. recovering and reutilizing, specific waste components. Rather, the classified light, combustible fraction is simply burned.